1. Field of the Invention
The present invention relates, generally, to a system and method for diagnosing the state of a fuel cell stack and for controlling a fuel cell system and, more particularly, to a method for diagnosing the state of a fuel cell stack and for controlling a fuel cell system, by which the water content of a fuel cell stack, such as dried-out/flooded states (e.g., water drainage), is determined, using the magnitude of impedance of a multi-frequency current applied to the fuel cell stack.
2. Description of the Related Art
A fuel cell vehicle includes a fuel cell stack, used as a power source, in which multiple fuel cells are combined, a fuel supply system that provides hydrogen to the fuel cell stack, an air supply system that provides oxygen as an oxidant necessary for an electrochemical reaction, and a water and heat management system that adjusts the temperature of the fuel cell stack. The fuel supply system decompresses compressed hydrogen stored in a hydrogen tank and provides the decompressed hydrogen to the fuel electrode (anode) of the stack, and an air supply system provides air suctioned from exterior by an air blower to the air electrode (cathode) of the stack.
When hydrogen and oxygen are provided to the fuel electrode and the air electrode, respectively, hydrogen is divided into a proton and an electron by a catalyst at the fuel electrode, and the proton travels to the air electrode through an electrolyte membrane while the electron is drawn from the anode to the cathode through an external circuit, thus generating electric power. Once reaching the air electrode, the proton is reunited with the electron and reacts with oxygen to create water, thus also generating electric power. In other words, a combination of the electrochemical oxidation of hydrogen at a fuel electrode and the electrochemical reduction of oxygen at an air electrode induces electrons to continuously move from the anode to the cathode, with the concomitant generation of electric power and heat. Additionally, the electrochemical reaction of hydrogen with oxygen generates water vapor or liquid phase water.
An emission device is provided as a drain during the generation of electric power in the fuel cell stack, i.e., the by-products, such as water vapor, water, and heat, and unreacted reactants, such as hydrogen and oxygen. Gas such as water vapor, hydrogen and oxygen is discharged to the atmosphere through a ventilation hood. Components for operating a fuel cell, including an air blower, a hydrogen recirculation blower, a water pump, and the like, are connected to a main bus to facilitate the operation of a fuel cell. The main bus may also be connected with various relays for facilitating power interruption and connection, and a diode for preventing a back current to the fuel cell.
Dry air provided by an air blower is humidified using a humidifier and is provided to the air electrode of a fuel cell stack while exhaust gas from the air electrode is delivered in a humidified state via the water generated within to the humidifier, and is used to humidify the dry air provided by an air blower. When a fuel cell stack is in a dried-out or flooded state, the fuel cell stack decreases in output power and the time required to recover the output power to a proper level increases. In addition, continuation of a dried-out or flooded state may result in a decrease in the durability and lifespan of the fuel cell stack. Therefore, it is necessary to accurately diagnose the state of a fuel cell stack, including dry-out or flooding (e.g., a water drainage state), and to ensure a rapid recovery of the fuel cell stack by performing recovery operations of the stack according to the diagnosed state.
A related art discloses a method for diagnosing the water content of a fuel cell stack by measuring an alternating current (AC) impedance of the fuel cell. In other words, when a current impedance is substantially constant, a fuel cell may be diagnosed as being normal. Further, when the fuel cell exhibits significant variation in current impedance, a flooding of the fuel cell may be determined. A gradually increasing current impedance may indicate that fuel cell is in a dried-out state. This disclosed method requires substantial amount of time to detect flooding since the method determines flooding by determining a variation of an impedance value; however, this requires measuring two or more impedance values over time and computing a variation of the impedance values. Additionally problematic is that measuring an impedance frequency range of several hundred Hz may result in a decrease in precision of measurement.
Another related art discloses a method for preventing a fuel cell stack from drying out by determining the relative humidity of the fuel cell stack using a relative humidity map based on measurement data regarding respective temperatures of a blower outlet, a humidifier inlet, and a coolant outlet, and by reducing an air flow and increasing a driving pressure when the relative humidity is determined to increase. This method is, however, problematic in that because humidity inside a fuel cell stack is highly susceptible to various factors, including the temperature and humidity of air suctioned into the fuel cell stack, the temperature and humidity in an air outlet, the temperature and humidity of provided hydrogen, the amount of product water of the fuel cell stack, and the temperature of the fuel cell stack itself. Estimating the relative humidity from data regarding respective temperatures of the air blower outlet, the humidifier inlet, and the coolant outlet has a reduced degree of accuracy.
Furthermore, another related art discloses a method of recovering a water balance in a fuel cell stack. When a dry-out situation occurs in the fuel cell stack in which the water balance in a fuel cell is less than a predetermined value, the fuel cell stack is recovered by restricting the output of the fuel cell through sequential processes of reduction of an air stoichiometric ratio, reduction of fuel electrode pressure, increase of an fuel electrode circulation volume, and increase of air pressure in that order. However, the sequential processes suffer from the disadvantage of requiring a substantial amount of time to recover a fuel cell stack in a dried-out state.